1. Field of the Invention
The present invention is directed to an arc welding apparatus. More particularly, the present invention is directed to an improved arc welding apparatus of the type wherein physical contact of a welding electrode with a work surface is mechanically sensed.
2. Brief Description of the Prior Art
Arc welding apparatus are well known in the prior art. As is well appreciated by those skilled in the art, during a welding operation the welding electrode must be at a predetermined distance from the work surface. This distance is usually determined principally on the basis of the nature of the electrode, the workpiece, the intensity of the electric arc employed for the welding, and the nature of the desired weld.
In order to position the electrode at the desired predetermined distance from the work surface (set in advance by an operator), welding apparatus of the prior art usually employ a lead screw actuated mounting assembly or carriage for the electrode. The lead screw is rotated by an electric motor and, depending on the direction of rotation of the motor, a threaded sleeve mounted to the lead screw carries the electrode towards or away from the work surface.
Because it is important to establish a reference point regarding the location of the electrode relative to the workpiece, the electrical resistance or impedance between the electrode and the workpiece is monitored in the prior art. The electrode is considered to be at zero distance from the workpiece when the monitored resistance or impedance falls below a predetermined threshold value. When this occurs, an electronic control unit causes the electric motor to reverse to carry the electrode away from the workpiece to a predetermined distance.
The above-noted prior art device and method for establishing contact of the electrode with the workpiece, however, suffers from the following disadvantage. When the surface of the workpiece is corroded, rusty or unclean, a relatively large electric resistance or impedance may exist between the electrode and the workpiece. In such a case the prior art "resistance or impedance monitoring" device does not "sense" the physical contact of the electrode with the workpiece, and the electric motor may continue to attempt to drive the lead screw actuated carriage assembly further towards the workpiece.
An improved arc welding device which overcomes the above-noted disadvantage of the formerly-mentioned prior art arc welding devices is described in a copending application for U.S. patent, titled "Arc Welding Apparatus," having Ser. No. 372,193 filed Apr. 26, 1982. The device described in the copending application operates on the principle that an electric switch operatively associated with a carriage assembly bearing the electrode is mechanically tripped substantially in the instance when the electrode physically contacts the workpiece. Tripping of the switch, then, causes the electric motor to reverse rotation for a predetermined time period whereby the electrode is carried to a predetermined distance from the workpiece.
Specific construction of this device involves a lead screw which is mounted in a housing for rotation and for limited upward sliding movement relative to the housing. The upward sliding movement occurs, however, only when the lead screw actuated carriage is in the process of moving towards the workpiece and further movement of the electrode (and therefore of the carriage) is impossible bcause of physical contact of the electrode with the workpiece.
Although the apparatus described in the above-noted copending application works well, further improvements are desirable in its specific construction to render the apparatus even more reliable of operation and economical or manufacture. The present invention is such an improvement.